Ionic bulk polymerization of styrene is not widespread. Typically, styrene polymers are prepared by thermal or radical-initiated (batch or continuous) bulk polymerization; solution polymerization; emulsion polymerization; or suspension polymerization.
The bulk polymerization in a spray-tower of conjugated diolefins for the production of rubbers and of thermoplastic polymers is known from U.S. Pat. No. 3,350,377. The patent discloses the anionic polymerization of, e.g., 1,3-butadiene, optionally with other polymerizable monomeric materials such as, among others, styrene and methylstyrene. Additionally, the patent mentions the Ziegler-Natta catalysed polymerization of 1-olefins (propylene, butylene, isobutylene, styrene, methylstyrene, etc.). This process comprises: (a) maintaining the monomeric liquid material at a first temperature at which polymerization will not occur; (b) admixing a suitable polymerization catalyst (such as butyllithium in the case of (co)polymerizing conjugated diolefins and Ziegler-Natta catalysts in case of 1-olefins) with said monomeric material at said first temperature; (c) spraying the resulting admixture downwards in the upper portion of a polymerization zone; (d) passing the vapor from a volatile hydrocarbon quench liquid heated to a second temperature at least above the polymerization initiation temperature of said admixture upwardly in said polymerization zone, thereby heating the downwards flowing spray of said admixture, and thus initiating polymerization; (e) permitting the polymer to quench and settle in the lower portion of said polymerization zone containing the relatively cooler quench liquid thereby vaporizing at least a portion of said quench liquid to produce said vapors that pass upwardly in said polymerization zone; and (f) withdrawing polymer solids from said lower portion.
The disadvantage of this process concerns the removal of the quenching solvent, that is essential to initiate the polymerization and prevent run-away reactions, from the polymerized droplets in a subsequent drying zone. Moreover, the organometal compounds mentioned as (part of the) catalyst for polymerizing the 1-olefins are simply too hazardous to handle in such process (note that at the reaction conditions the alkylaluminium compounds are relatively volatile and extremely air and moisture sensitive). Finally, the patent specification merely mentions the possibility of polymerizing 1-olefins, without any reference to the molecular weight of the so produced thermoplastic polymer or its residual monomer content. This is not entirely surprising as the patent is primarily concerned with the production of rubbers whereas the remaining monomers are anyway removed in the drying zone.
U.S. Pat. No. 3,644,305 discloses a process that is coined "spray-polymerization". It is similar to the process in U.S. Pat. No. 3,350,377, however, concerning radical-initiated, solution polymerization of vinyl monomers, in particular water-soluble, ethylenically-unsaturated monomers. The disadvantages of this process, if applied in respect of the production of styrene polymers, concern the presence of the solvent and the lesser stability of the radical polymerized polymer. Moreover, according to the patent specification, the time of contact is relatively short and therefore spray polymerization provides only relatively low molecular weight polymers.
The inventors set out to develop a process for anionically preparing styrene polymers, as anion-initiated polymerization more readily allows the production of copolymers, and provides more stable polymers (avoiding or at least reducing the need for light stabilizers). The ideal process should moreover be able to afford a wide range of different grades, produced in high yield and preferably in continuous mode at low fixed and variable costs. The process should also be highly flexible, avoiding the production of twilight materials. Naturally, problems associated with residual styrene monomer or viscosity of the styrene polymer should be evaded.